Self-expanding stent with enhanced delivery precision and stent delivery system

ABSTRACT

The invention is directed to a self-expanding stent for implantation into a body lumen, such as an artery. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common longitudinal stent axis and interconnected by a plurality of interconnecting members placed on the stent in a collinear arrangement such as to create at least one continuous spine which extends along the length of the stent. The invention is also directed to a stent delivery system for implantation of a stent in a vessel which includes an outer tubular member having a restraining sheath and an inner tubular member having a distal end which has a compressed stent mounted thereto. The proximal end of the inner tubular member is connected to a housing assembly which prevents the inner tubular member from moving when the outer tubular member is retracted to deploy the stent. The proximal end of the outer tubular member is attached to a pull-back handle which is slidably mounted on the base of the housing assembly. When the pull-back handle is retracted, the restraining sheath is retracted to deploy the sheath, while the inner tubular member remains stationary.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to expandable endoprosthesisdevices, generally called stents, which are adapted to be implanted intoa patient's body lumen, such as a blood vessel, to maintain the patencythereof, along with systems for delivering and deploying such stents.Stents are particularly useful in the treatment and repair of bloodvessels after a stenosis has been compressed by percutaneoustransluminal coronary angioplasty (PTCA), percutaneous transluminalangioplasty (PTA), or removed by atherectomy or other means, to helpimprove the results of the procedure and reduce the possibility ofrestenosis.

[0002] Stents are generally cylindrically shaped devices which functionto hold open and sometimes expand a segment of a blood vessel or otherarterial lumen, such as coronary artery. Stents are usually delivered ina compressed condition to the target site and then deployed at thatlocation into an expanded condition to support the vessel and helpmaintain it in an open position. They are particularly suitable for useto support and hold back a dissected arterial lining which can occludethe fluid passageway there through.

[0003] A variety of devices are known in the art for use as stents andhave included coiled wires in a variety of patterns that are expandedafter being placed intraluminally on a balloon catheter; helically woundcoiled springs manufactured from an expandable heat sensitive metal; andself-expanding stents inserted into a compressed state for deploymentinto a body lumen. One of the difficulties encountered in using priorart stents involve maintaining the radial rigidity needed to hold open abody lumen while at the same time maintaining the longitudinalflexibility of the stent to facilitate its delivery and accommodate theoften tortuous path of the body lumen.

[0004] Prior art stents typically fall into two general categories ofconstruction. The first type of stent is expandable upon application ofa controlled force, often through the inflation of the balloon portionof a dilatation catheter which, upon inflation of the balloon or otherexpansion means, expands the compressed stent to a larger diameter to beleft in place within the artery at the target site. The second type ofstent is a self-expanding stent formed from shape memory metals orsuper-elastic nickel-titanum (NiTi) alloys, which will automaticallyexpand from a compressed state when the stent is advanced out of thedistal end of the delivery catheter into the blood vessel. Such stentsmanufactured from expandable heat sensitive materials allow for phasetransformations of the material to occur, resulting in the expansion andcontraction of the stent.

[0005] Details of prior art expandable stents can be found in U.S. Pat.No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.);U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665(Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882(Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No.5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat.No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S.Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.

[0006] Further details of prior art self-expanding stents can be foundin U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003(Wolff, et al.), which are hereby incorporated by reference.

[0007] Some prior art stent delivery systems for implantingself-expanding stents include an inner lumen upon which the compressedor collapsed stent is mounted and an outer restraining sheath which isinitially placed over the compressed stent prior to deployment. When thestent is to be deployed in the body vessel, the outer sheath is moved inrelation to the inner lumen to “uncover” the compressed stent, allowingthe stent to move to its expanded condition. Some delivery systemsutilize a “push-pull” type technique in which the outer sheath isretracted while the inner lumen is pushed forward. Still other systemsuse an actuating wire which is attached to the outer sheath. When theactuating wire is pulled to retract the outer sheath and deploy thestent, the inner lumen must remain stationary, preventing the stent frommoving axially within the body vessel.

[0008] However, problems have been associated with prior art deliverysystems. For example, systems which rely on a “push-pull design” canexperience movement of the collapsed stent within the body vessel whenthe inner lumen is pushed forward which can lead to inaccuratepositioning and, in some instances, possible perforation of the vesselwall by a protruding end of the stent. Systems which utilize anactuating wire design will tend to move to follow the radius ofcurvature when placed in curved anatomy of the patient. As the wire isactuated, tension in the delivery system can cause the system tostraighten. As the system straightens, the position of the stent changesbecause of the length of the catheter no longer conforms to thecurvature of the anatomy. This change of the geometry of the systemwithin the anatomy can also lead to inaccurate stent positioning.

[0009] Another difficulty which can be encountered with some existingself-expanding stents is the fact that the length of the stent canshorten dramatically during deployment, making it difficult to preciselyposition the stent within the artery. Since proper positioning of thestent is critical to the performance of the stent, it is imperative thatthe physician know the exact length and diameter that the stent willexpand to upon deployment. A self-expanding stent which shortens inlength upon radial expansion of the device can cause problems to thephysician attempting to accurately position the stent within the targetsite. Additionally, some existing self-expanding stents can store energyaxially as the outer restraining sheath is retracted. Frictional forcegenerated as the outer sheath is retracted over the self expanding stentcan cause the stent to act somewhat like a spring, storing energy as thefrictional force acts on the stent. The stored energy is released as thestent expands beyond the end of the sheath, and this release of energycan cause the stent to move or “jump” from the desired position,resulting in inaccurate placement. The amount of energy stored isdependent on the flexibility of the stent and the friction between thestent and the outer sheath.

[0010] The above-described stent delivery systems also can be somewhatdifficult to operate with just one hand, unless a mechanical advantagesystem (such as a gear mechanism) is utilized. Often, deployment withone hand is desirable since it allows the physician to use his/her otherhand to support a guiding catheter which is also utilized during theprocedure, allowing the physician to prevent the guiding catheter frommoving during deployment of the stent. Neither of the above-describedprior art stent delivery systems prevents any axial movement of thecatheters of the system during stent deployment. Even a slight axialmovement of the catheter assembly during deployment can cause inaccurateplacement of the stent in the body lumen.

[0011] What has been needed and heretofore unavailable is aself-expanding stent which has a high degree of flexibility so that itcan be advanced through tortuous passageways of the anatomy and can beexpanded up to its maximum diameter with minimal, or no longitudinalcontraction, and yet have sufficient mechanical strength to hold thebody lumen open. The self-expanding stent should also store little or noenergy during sheath retraction to prevent “jumping” of the stent fromoccurring to allow for more accurate positioning within the body lumen.Also, there is a need for a stent delivery system which facilitatesminimal movement during stent deployment, provides accurate stentplacement, and provides single handed operation by the physician. Thepresent inventions disclosed herein satisfy all of these needs.

SUMMARY OF INVENTION

[0012] The present invention is directed to a self-expanding stenthaving a configuration which permits the stent to be expanded radiallyto larger diameters while preventing longitudinal shortening of thestent during expansion. As a result, the present invention provides astent which maintains a constant length from its fully compressedcondition all the way through to its fully expanded condition. Aself-expanding stent made in accordance with the present inventionprovides for more accurate placement during the delivery of the stent tothe target site in the body lumen. The stent remains relatively flexiblealong its longitudinal axis in order to facilitate delivery throughtortuous body lumens, but is strong enough radially in its expandedcondition to maintain the patency of the body lumen, such as an arteryor other vessel, when implanted therein.

[0013] The stent of the present invention also minimizes the potentialfor storing energy as the outer restraining sheath of the stent deliverycatheter is retracted over the compressed stent. The structure of thestent made in accordance with the present invention stores little or noenergy during deployment, reducing the likelihood that the stent will“jump” off of the delivery catheter when the last few rings of the stentare released. As a result, a smooth and controlled deployment can beachieved when utilizing the stent of the present invention. This stentdesign results in a low profile device which maintains good flexibilityto reach even distal lesions.

[0014] The stent of the present invention includes a plurality ofadjacent cylindrical elements (also referred to as “rings”) which areindependently expandable in the radial direction and arranged along acommon longitudinal axis. The cylindrical elements are formed in anirregular serpentine wave pattern transverse to the longitudinal axisand continuing in a plurality of alternating peaks and valleys. Eachcylindrical element is connected to an adjacent cylindrical element byat least one interconnecting member which is aligned longitudinally withanother interconnecting member to create a continuous spine which runsthe length of the stent to prevent any significant stent shorteningduring expansion. The continuous spine also helps prevent unwantedstorage of energy in the stent as the outer restraining sheath of thedelivery catheter is retracted to deploy the stent.

[0015] In one preferred embodiment of the present invention, eachcylindrical element is connected to an adjacent cylindrical element bythree interconnecting members which are circumferentially positioned 120degrees apart. In this embodiment, the interconnecting members arealigned to form three continuous spines along the length of the stent,again to prevent any significant shortening of the stent during radialexpansion and to prevent unwanted storage of energy as the outerrestraining sheath is retracted for deployment.

[0016] The presently preferred structure for the expandable cylindricalelements which form the stent of the present invention generally has acircumferential serpentine pattern along a plurality of alternatingpeaks and valleys. Each cylindrical element contains three (3) “W” andthree (3) “U” shaped patterns which form the valleys of the stent. Each“W” and “U” shaped valley is connected by an inverted “U” shaped patternwhich forms the peaks of the cylindrical element. As the stent expands,the “W”, and “U” and inverted “U” patterns open circumferentially, withthe interconnecting members maintaining the spacing between eachcylindrical element. To minimize the gaps between the struts when thestent is expanding, each serpentine cylindrical element is designed toextend into the space between the “W”, the “U” and the inverted “U” ofan adjacent cylindrical element. The interconnecting members ensureminimal longitudinal contraction during radial expansion of the stent inthe body vessel. Preferably the serpentine patterns have varying degreesof curvature in the regions of the peaks and valleys and are adapted sothat radial expansion of the cylindrical elements are generally uniformaround their circumferences during expansion of the stent from thecontracted condition to the expanded condition.

[0017] The resulting stent structure is a series of radially expandablecylindrical elements that are spaced longitudinally close enough so thatsmall dissections in the wall of a body lumen may be pressed back intoposition against the luminal wall, yet does not compromise thelongitudinal flexibility of the stent both when being negotiated throughthe body lumens in the unexpanded state and when expanded into position.The serpentine patterns allow for even expansion around thecircumference by accounting for the relative differences in stresscreated by the radial expansion of the cylindrical elements. Each of theindividual cylindrical elements may rotate slightly relative to theiradjacent cylindrical elements without significant deformation,cumulatively providing a stent which is flexible along its length andlongitudinal axis, but which is still very stable in the radialdirection in order to resist collapse after expansion. The openreticulated structure of the stent results in a low mass device. It alsoenables the perfusion of blood over a large portion of the arterialwall, which can improve the healing and repair of a damaged arteriallining.

[0018] The stent of the present invention can be laser cut from a tubeof super elastic nickel titanium (Nitinol) whose transformationtemperature is below body temperature. All of the stent diameters arecut with the same stent pattern, and the stent is expanded and heattreated to be stable at the desired final diameter. The heat treatmentalso controls the transformation temperature of the Nitinol such thatthe stent is super elastic at or below body temperature. The stent iselectro polished to obtain a smooth finish with a thin layer of titaniumoxide placed on the surface. The stent is usually implanted into thetarget vessel which is smaller than the stent diameter so that the stentapplies a force to the vessel wall to keep it open.

[0019] After the stent is expanded, some of the peaks and/or valleysmay, but not necessarily, tip outwardly and embed in the vessel wall.Thus, after expansion, the stent might not have a smooth outer wallsurface. Rather, they might have small projections which embed in thevessel wall and aid in retaining the stent in place in the vessel.

[0020] The elongated interconnecting members which interconnect adjacentcylindrical elements should have a transverse cross-section similar tothe transverse dimensions of the undulating components of the expandablecylindrical elements. The interconnecting members may be formed in aunitary structure with the expandable cylindrical elements formed fromthe same intermediate product. The stent could also be made from a sheetof material with the pattern of the cylindrical elements andinterconnecting elements cut by a laser. The sheet could then be formedinto a cylinder by welding a longitudinal seam using laser welding orother known techniques.

[0021] Preferably, the number and location of the interconnectingmembers can be varied in order to develop the desired longitudinalflexibility provided by the rings in the stent structure both in thecompressed condition as well as in the expanded condition. Theseproperties are important to minimize alteration of the naturalphysiology of the body lumen into which the stent is implanted and tomaintain the compliance of the body lumen which is internally supportedby the stent. Generally, the greater the longitudinal flexibility of thestents, the easier and the more safely they can be delivered to theimplantation site, especially where the implantation site is on a curvedsection of a body lumen, such as a coronary artery or a peripheral bloodvessel, and especially saphenous veins and larger vessels. The number ofspines formed by the collinear arrangement of interconnecting elementscan vary from one to as many as can be reasonably placed on the stent,however, for minimal energy storage with maximum flexibility, two tofour spines are preferred.

[0022] The stent of the present invention is particularly useful forimplantation in body lumens which are located along the outer portionsof the body where external forces could possibly be applied to thestent. For example, the stent of the present invention is particularlyadvantageous of implantation in the carotid arteries which aresusceptible to external forces. Since the Nitinol stent is crushedresistant, it will spring back to its original expanded condition evenafter an external force is applied to it. As a result, there is lesslikelihood that the stent would be deformed or crushed by an externalforce. Additionally, due to the springy and softer composition of thestent, there is less likelihood that the struts of the stent would cutinto the underlying plaque build-up upon application of a force whichmay otherwise create small pieces of plaque that would enter thebloodstream.

[0023] The present invention also is directed to a stent delivery systemwhich can be used to provide accurate deployment of an self-expandingstent into a target site in a patient's body lumen. The stent deliverysystem in accordance with the present invention incorporates uniquefeatures which facilitates minimal movement during stent deployment,accurate stent placement, and single-handed system operation. The stentdelivery system can be used to deploy the novel self-expanding stentdisclosed herein, or any self-expanding stent.

[0024] One preferred embodiment of a stent delivery system made inaccordance with the present invention includes an elongated catheterbody having a proximal and distal end. The elongated catheter body ismade up of an inner tubular member which extends within an outer tubularmember in a coaxial arrangement. The outer tubular member has arestraining sheath at its distal end which holds the stent, which ismounted on the inner tubular member, in its compressed delivery positionuntil ready for deployment. The outer tubular member and restrainingsheath are retractable to release the compressed stent to its expandedcondition. The proximal ends of the inner and outer tubular members areconnected to a housing assembly which provides a manual mechanism forretracting the restraining sheath and immobilizing the inner tubularmember, preventing it from moving relative to the restraining sheathduring stent deployment. The proximal end of the outer tubular member isattached to a pull-back handle located on the housing assembly which ismoved by the physician in order to retract the restraining sheath inorder to deploy the compressed stent. A luer fitting attached to theproximal end of the inner tubular member is rigidly fixed to the housingbase to prevent the inner tubular member from moving when the outertubular member is retracted.

[0025] The inner tubular member has a guide wire lumen which extendsfrom the distal end of the inner tubular member to the proximal end toallow a guide wire to be used to advance the elongated catheter body tothe target area in the body lumen in an “over the wire” technique. Inthis regard, the catheter stent assembly can be introduced within thepatient's vasculature in a conventional Seldinger technique through aguiding catheter. The distal end of the inner tubular member includes asoft, low profile tip assembly with a radiopaque marker. An additionalradiopaque marker is placed proximally to the collapsed stent.

[0026] In a preferred embodiment of the present invention, the innertubular member is made with three (3) coaxial layers of materials. Theinner most layer is the guide wire lumen (described above) which runsthe entire length of the catheter body. A second layer of the innertubular member is composed of a proximal portion made from stainlesssteel hypotube and a distal reinforcing portion which can be made from amaterial with high compressive strength such as polyetheretherketone(PEEK). The outermost part of the inner tubular member is a thin layershrink tubing.

[0027] In a preferred embodiment, the tip assembly of the inner tubularmember includes a tubular element made from a piece of stainless steelhypotube to which a wound coil is welded. The coil and the distal end ofthe tubular element are encased in molded urethane. The distal end ofthe urethane body is loaded with radiopaque tungsten making the tipassembly radiopaque. The proximal end of the tubular segment can includecircumferential slots which are cut into the proximal end to provide achannel which allows air and fluid to escape when the catheter assemblyis flushed to evacuate air from the system.

[0028] The housing assembly of the stent delivery system is designed sothat the operator retracts only the outer restraining sheath while theinner tubular member remain stationary. Due to the unique design of thehousing assembly, the physician pushes down on the housing assemblyduring deployment and not forward. This prevents the inner tubularmember assembly from moving forward toward the patient. The housingassembly includes a uniquely curved base which has a contour whichconforms to the patient's leg. The design of the housing allows thesystem to be operated by just one hand, freeing the physician's otherhand for other purposes, such as stabilizing the guiding catheter duringstent deployment.

[0029] The stent delivery system of the present invention also includesa unique flushing system which is used to evacuate air from the system.The flushing system consists of small openings extending through theinner tubular member near the end of the proximal portion where it meetsthe distal portion of the inner member. The openings are drilled throughthe guide wire lumen to effectively open up a passageway from the guidewire lumen to the annular space formed between the inner tubular memberand the outer tubular member. A syringe is attached to the luer fittingat the housing assembly and sterile fluid is pumped into the guide wirelumen in order to flush air from the system. A mandrel placed in theguide wire lumen at the tip assembly blocks the flow of the sterilefluid through the distal tip. The sterile fluid is thus forced to flowout of the small openings into the annular space formed between theinner tubular member and outer tubular member. The fluid flows past thecollapsed stent where the fluid will eventually escape either throughthe small circumferential slots cut into the tubular element of the tipassembly or from the sheath directly. Once fluid is observed drippingfrom the end of the restraining sheath, the mandrel can be removed sinceair has been evacuated from the system. Since the gap sizes are so smallbetween the various components, capillary force prevents air frominfiltrating the delivery system once the evacuation has been completed.

[0030] These and other advantages of the present invention becomeapparent from the following detailed description and the accompanyingexemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is an elevational view, partially in section, depicting theself-extending stent embodying features of the present invention whichis mounted on a stent delivery system made in accordance with thepresent invention and disposed within a vessel.

[0032]FIG. 2 is an elevational view, partially in section, similarly tothat shown in FIG. 1, wherein the stent is expanded within the vessel.

[0033]FIG. 3 is a plan view showing the housing assembly of the stentdelivery system shown in FIG. 1 in its locked position.

[0034]FIG. 4 is a plan view of the housing assembly of the stentdelivery system shown in FIG. 1 in its unlocked position.

[0035]FIG. 5 is a cross-sectional view of the housing assembly takenalong lines 5-5.

[0036]FIG. 6 is a cross-sectional view of the housing assembly takingalong lines 6-6.

[0037]FIG. 7 is an elevational view of the inner tubular member of thecatheter portion of the stent delivery system made in accordance withthe present invention.

[0038]FIG. 8 is an elevational view showing the housing assembly of thepresent invention being manually operated.

[0039]FIG. 9 is a plan view of a preferred embodiment of a flattenedstent of the present invention, which illustrates the serpentine patternwith the interconnecting members arranged collinearly to form acontinuous spine along the stent.

[0040]FIG. 10 is an enlarged partial view of the stent of FIG. 9depicting the serpentine pattern along the peaks and valleys which formthe cylindrical element of the state of the present invention.

[0041]FIG. 11 is a cross sectional view of the inner tubular membertaken along lines 11-11.

[0042]FIG. 12 is a cross sectional view of the catheter body shown inFIG. 1 taken along lines 12-12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The present invention is directed to a self-expanding stent withenhanced delivery precision and a stent delivery system for accuratelyplacing self-expanding stents into a target site in a body lumen. Whilethe present invention is described in detail as applied to the coronaryarteries of a patient, those skilled in the art will appreciate it thatit can also be used in other body lumens as well, peripheral arteriessuch as the carotid artery, and veins.

[0044] FIGS. 1-4 illustrate a self-expanding stent 10 incorporatingfeatures of the present invention. The stent 10 is mounted onto a stentdelivery system 11 which is also made in accordance with the presentinvention. The stent 10 generally comprises a plurality of radiallyexpandable cylindrical elements 12 disposed generally coaxially andconnected by interconnecting members 13 disposed between adjacentcylindrical elements 12. Additional details regarding the particularstructure and shape of the various elements making up the stent 10 areprovided below.

[0045] The stent delivery system 11 has an elongated catheter body 14for delivering and deploying the compressed stent 10 (as shown inFIG. 1) within an artery 15 or other vessel. The artery 15, as shown inFIGS. 1 and 2, has an area of treatment 16 which has just undergone anangioplasty procedure, or similar procedure, in which atheroscleroticplaque of a stenosis has been compressed against the inside wall 17 ofthe artery 15 to increase the diameter of the occluded area of artery15. The expanded stent 10 (shown in FIG. 2) is implanted within theartery 15 to help hold open the artery in this area and to help preventrestenosis.

[0046] The stent delivery system 11 includes a housing assembly 18attached to the proximal end 19 of the delivery catheter 14 which isused to manually deploy the compressed stent 10 mounted on the distalend 20 of the delivery catheter 14 into the diseased artery 15. Thedelivery catheter 14 includes an inner tubular member 21 which extendswithin an outer tubular member 22 in a coaxial arrangement. The innertubular member 21 has a luer fitting 23 attached at its proximal end 24which is rigidly attached to the base 25 of the housing assembly 18 toprevent the inner member 21 from moving relative to the outer member 22during stent deployment. The outer member 22 has a proximal end 26 whichis attached to a pull-back handle 27 which is designed to move axially(along the longitudinal axis of the delivery catheter 14) within thebase 25. At the distal end of the outer tubular member 22 is a flexiblerestraining sheath 29 which is welded or otherwise attached to theelongated shaft 28 of the outer tubular member 22. This restrainingsheath 29 is designed to hold the stent 10 in its compressed orcollapsed state and is retracted by moving the pull back handle 27 (inthe direction of the arrows 30 shown in FIG. 4) which moves therestraining sheath in a likewise fashion while maintaining the innertubular member 21 stationary during stent deployment.

[0047]FIG. 8 shows how the pull-back handle 27 of the housing assembly18 can be grasped by a single hand 31 of the physician to deploy thecollapsed stent 10. The housing assembly 18 includes a pair of thumbgrooves 32 which are located at the proximal end 33 of the base 25 andare adapted to receive the thumb of the physician when the stent is tobe deployed. The pull-back handle 27 includes a pair of recesses 34adapted for the fingers of the physician. The physician simply pullsback on the pull-back handle 27 to deploy the stent 10 once in properposition. Since the thumb grooves 32 are perpendicular to the axis ofthe restraining sheath 29, the physician can usually only push downwardon the base 25 of the housing assembly 18 and not forward. This helpsprevent the housing assembly 18 from moving forward, towards thepatient. By directing the force of the physician's hand down on the baseand away from the patient via the pull-back handle 27, rather thanforward, the distal end of the delivery catheter 14 should be preventedfrom moving within the artery to insure an accurate placement of thestent 10 in the body lumen. Since the stent delivery system 11 can beused with just one hand, the physician's other hand is free to performother tasks, such as stabilizing the guiding catheter used during theprocedure. By stabilizing the guiding catheter as well, enhancedaccuracy in deploying the stent can be obtained. Details concerningadditional features of the housing assembly 18 are provided below.

[0048] In one embodiment of the present invention, the inner tubularmember 21 is a composite structure formed from three coaxial layers ofmaterials, each material having a specific function. The innermost layeris a guide wire lumen 35 which runs the entire length of the deliverycatheter 14. This guide wire lumen 35 can be made from a material suchas a high density polyethylene (HDPE) or similar material which providesa low friction interface between the delivery catheter and the guidewire (not shown) which is also used in the procedure to advance thecatheter body 14 to the target site using over-the-wire techniques thatare well known in the art. For example, the guide wire lumen 35 can bemade from tubing which is compatible with a 0.014 inch guide wire for anover-the-wire configuration.

[0049] The application of tensile force to the shaft of the outertubular member 22 and restraining sheath 29 during stent deploymentcreates an equal and opposite compressive force on the inner tubularmember 21. For the restraining sheath 29 to retract (via the movement ofthe pull-back handle 27) without causing the rest of the deliverycatheter 14 to buckle, the inner tubular member 21 must possesssufficient column strength to prevent buckling or deformation.Otherwise, buckling or deformation to the inner tubular 21 can cause thedistal end 20 of the delivery catheter 14 to move within the artery,causing inaccurate deployment of the stent. Therefore, the second layerof the inner tubular member may be comprised of tubular elements whichpossess sufficient rigidity to prevent unwanted buckling or deformation,yet are flexible enough to track along the torturous anatomy to thetarget site.

[0050] In a preferred embodiment of the present invention, the secondlayer of the inner tubular member 21 includes a proximal portion 36 madefrom a stainless steel hypotube, or similar material, and a distalportion 37 comprising of more flexible material such as polyethereketone(PEEK) or similar material which possess excellent compressive strengthyet is reasonably flexible. The proximal portion 36 is made fromhypotube which provides maximum strength, but is fairly rigid. However,this is not a concern since this proximal portion 36 of the innertubular member 21 remains relatively straight within the guidingcatheter during the procedure. The distal portion 37, which isapproximately 15 centimeters in length, must exit the guiding catheterand track through the torturous anatomy to reach the target site.Therefore, this portion must possess sufficient compressive strength yetbe fairly flexible.

[0051] The outermost layer of the inner tubular member 21 may be madefrom a layer of shrink tubing 38 having low frictional characteristics.A suitable material would be linear low density polyethylene (LLDPE).The outer layer of shrink tubing 38 is utilized to reduce the amount offriction created when the outer tubular member 22 is retracted over thelength of the inner tubular member 21. The outer surface of the innertubular member 21 can also be coated with a silicone lubricant such asMicrogilde manufactured by Advanced Cardiovascular Systems, Inc., SantaClara, Calif., to further reduce the amount of frictional buildupbetween the outer tubular member 22 and inner tubular member 21.

[0052] A luer fitting 23 attached to the proximal portion 36 of theinner tubular member 21 is rigidly mounted to the base 25 of the housingassembly 18 to permanently secure the inner member to the housingassembly. The luer fitting 23 can be attached to the inner tubularmember 21 by trimming the guide wire lumen 35 at the proximal end andthen gluing the fitting 23 and proximal portion 36 together with asuitable adhesive. It should be appreciated that the mounting of theinner tubular member to the housing assembly 18 can be achieved in anynumber of ways without departing from the spirit and scope of thepresent invention.

[0053] The distal end 39 of the inner tubular member 21 includes a stentholder 40 upon which the compressed stent 10 is mounted. A tip assembly41 having a tapered configuration is located at the distal end of thedelivery catheter 14 to help crossing and areas of occlusions in thediseased artery. A tantalum marker 42 is attached to the proximal end ofthe stent holder 40 by adhesive or other means. The tantalum marker 42is radiopaque and is used to locate the proximal end of the stent 10. Inaddition, the marker 42 is larger than the inner diameter of thecompressed stent 10 to provide an abutting surface for the stent 10 topush against when the restraining sheath 29 is being retracted. Thestent holder 40 can be made from a piece of tubing which correctly sizesthe mismatch between the inner diameter of the collapsed stent 10 andthe rest of the inner tubular member 21. For example, the stent holdercan be made from a composite material having a mix of 75% LLDPE whichmakes it soft and flexible with 25% HDPE to improve process ability. Thestent holder 40 has a tapered distal tip 43 to facilitate attachment tothe tip assembly 41. The stent holder 40 can be glued directly onto theguide wire lumen 35 and is encased under the layer of shrink tubing 38which forms the outermost layer of the inner tubular member 21.

[0054] The tip assembly 41 is made from a tubular element 44 made from asmall segment of stainless steel hypotube which has a tapered wound coil45 welded to the distal end of the tubular element 44. The coil 45 andthe distal portion of the mounting segment are incased in moldedurethane to form the tip component 46. A radiopaque tungsten element 47is placed at the distal end of the tip component 46. The guide wirelumen 35 extends through the tip component to the distal tip 48. Anopening (not shown) at the distal end of the assembly tip 41 permits theguide wire to advance therethrough to allow the delivery catheter 14 totrack along the wire into the diseased artery.

[0055] The tubular element 44 has a number of circumferential slots 49cut into the proximal end of the element 44. The slots 49 provide achannel which allows fluid to escape when the device is being flushed toevacuate air from the delivery system. The proximal end 50 of thetubular element 44 abuts the distal end of the stent holder 48 and ispartially covered by the restraining sheath 29. At least a small segmentof the slots 49 should be unsheathed to allow the flushing fluid and airto escape from the system during the air evacuation step. The tipcomponent 46 includes a shoulder 51 which is raised from the outersurface of the tubular element 44 so that the distal end 52 of therestraining sheath 29 remain flush with the tip component 46. Thisparticular configuration prevents the distal end 52 of the restrainingsheath 29 from being exposed while the delivery catheter is beingmaneuvered through the curves of the anatomy.

[0056] The elongated shaft 28 of the outer tubular member 22 can be madefrom a material such as cross-linked HDPE. The restraining sheath 29 canbe made from a material such as polyolifin which is welded or otherwiseattached to the shaft 28 of the outer tubular member. A material such aspolyolifin is used since it has sufficient strength to hold thecompressed stent and has relatively low frictional characteristics tominimize any friction between the stent 10 and the sheath 29. Frictioncan be further reduced by applying a coat of silicone lubricant, such asMicrogilde, to the inside surface of the restraining sheath 29 beforethe stent 10 is loaded onto the stent holder 40.

[0057] Referring now to FIGS. 3-6, the housing assembly 18 is shownincluding a lock mechanism 53 which is designed to maintain thepull-back handle 27 in its forward position until the stent is ready tobe deployed. The base includes a cover 54 which extends from the distalend of the base 25 to its proximal end. This cover 54 includes anopening 55 for receiving the lock mechanism 53. The lock mechanism 53 isoperated by simply grasping the control knob 56 and rotating it toeither the locked or unlocked position. FIGS. 3 and 5 show the lockmechanism 53 in the locked position. In the locked position, a shoulderportion 57 of the lock mechanism 53 comes in contact with a raisedprojection 58 formed on the pull-back handle 27. The shoulder portion 57includes a slotted opening 59 through which the raised projection 58slides when the pull-back handle 27 is retracted to deploy the stent.The shoulder portion 57 abuts the raised projection 58 preventing itfrom moving passed it since the slotted opening 59 is oriented 90° outof phase with the raised protection 58. Referring now to FIGS. 4 and 6,which show the lock mechanism in the unlocked position, the slottedopening 59 on the shoulder 57 is now aligned with the raised projection58 to allow it to pass therethrough. In the open position shown in FIG.4 and 6, the lock mechanism 53 allows the pull-back handle 27 to bepulled back in direction of arrow 30, which retracts the restrainingsheath to deploy the compressed stent.

[0058] The base 25 of the housing assembly 18 includes a slotted channel60 which is adapted to receive the central section 61 of the pull-backhandle 27. The central portion 61 includes an opening 62 through whichthe proximal portion 36 of the inner tubular member 21 extends to alocation where the luer fitting 23 is rigidly mounted in a recess (notshown) or similar mounting element on the base 25. The proximal end 26of the outer tubular member 22 is affixed to the front plate 63 of thepull-back handle 27 so that as the pull-back handle is retracted, theouter member 22 and restraining sheath 29 are likewise retracted,accordingly while the inner member 21 remains stationary.

[0059] As can be seen in FIGS. 5 and 6, the base 25 has an uniquecontour which increases the surface area of the base and is contoured tofit the patient's leg. Thus, during the procedure, the physician canplace the housing assembly 18 directly onto the leg of the patient whereit should remain stationary as the sheath is being retracted. The uniquedesign of the housing assembly permits the physician to uses just onehand to retract the pull-back handle 27 to deploy the compressed stentinto its expanded condition without the worry of possibly moving thedelivery catheter during the deployment process.

[0060] The stent delivery system of the present invention also includesa unique flushing system which is used to evacuate air from the system.It is important to evacuate air from the system when the stent deliverysystem is being used to place a stent in the carotid artery since it isundesirable to have even a small air bubble ever the arteries in thebrain. In other instances, it may be desirable to have a fluid preplacedinto the system to prevent the possible accumulation of blood betweenthe retractable sheath and the innertubular member since stagnated bloodhas the tendency to coagulate and cause thrombosis. For these reasons,it may be beneficial to pre-flush the system before placing the deliverycatheter in the patient.

[0061] Referring now to FIGS. 7, 11 and 12, the flushing system consistsof openings 64 extending through the inner tubular member 21 in the areaof where the proximal portion meets the distal portion of the innermember (FIG. 7). The openings 64 are drilled through to the guide wirelumen 35 to effectively open up a passageway from the guide wire lumen35 to the annular space formed between the inner tubular member 21 andthe outer tubular member 22. A syringe is attached to the luer fitting23 of the housing assembly 18 and sterile fluid is pumped into the guidewire lumen 35 in order to flush air from the system. A mandrel (notshown) placed in the guide wire lumen 35 at the tip assembly 41 blocksthe flow of the sterile fluid through the distal tip. The sterile fluidis thus forced to flow out of the small openings 64 into the annularspace formed between the inner tubular member and outer tubular member.The fluid eventually flows past the collapsed stent (FIG. 12) where thefluid and any air in the system will escape through the smallcircumferential slots 49 cut into the tubular element 44 of the tipassembly 41. Once fluid is observed dripping from the distal end 52 ofthe restraining sheath 29, the mandrel is removed since air has beenevacuated from the system. Since the gap sizes are so small between thevarious components, capillary force prevents air from infiltrating thedelivery system once the evacuation has been completed.

[0062] Referring now to FIGS. 9 and 10, a preferred embodiment of thestent 10 of the present invention is shown. As can be seen in FIG. 10,the cylindrical element 12 of stent 10 illustrates the serpentinepattern having a plurality of peaks and valleys which aid in the evendistribution of expansion forces. At this embodiment, theinterconnecting members 13 serve to connect adjacent valleys of eachadjacent cylindrical element 12 as described above. The various peaksand valleys generally have U, Y and W and inverted U shapes, in a repeatpattern to form the cylindrical element 12. During expansion, doubledcurved portions (W) 70 located in the region of the valley whereinterconnecting members 13 are connected, have the most mass and are thestiffest structure during deformation, while peak portions (inverted U)72 are the least stiff, and valley portions (U) 74 have an intermediatestiffness. In the embodiment shown in FIGS. 9 and 10, there are threerepeating patterns of peaks and valleys in each cylindrical element 12,which allows the stent to be crimped to a very small profile. Each peakportion (inverted U) 72 has a shoulder portion 75 which has a differentradius of curvature than the radius of curvature for the valley portions(U 74) and peak portion (inverted U) 72. This shoulder region 75provides a transition region between the peak portion (inverted U) 72and the valley portions (U) 74 and double curved portion (W) 70 to allowadjacent cylindrical elements to overlap and thereby better support theartery walls with smaller gaps between stent struts. In this manner, theshoulder portion 75 provides more dense coverage of the serpentinepattern of the cylindrically element to create a fairly uniform strutpattern which fully supports the walls of the diseased artery. For thisreason, there are no or few areas of the stent wall which do not havestruts for supporting the walls of the artery.

[0063] Each interconnecting member 13 is aligned collinearly with eachother to form a continuous spine 76 which extends along the length ofthe stent 10. This continuous spine 76 prevents the stent fromshortening longitudinally when the cylindrical elements 12 are expandedradially. The spine 76 also helps prevent the stent from storing energyas the restraining sheath 29 is retracted over the stent duringdeployment. As a result, the stent 10 will not “jump” off the stentholder 40 as the last few cylindrical elements 12 are released by therestraining sheet 29. Therefore, more accurate deployment of the stentcan be achieved. The number and location of the interconnecting members13 can be varied in order to develop the desired longitudinalflexibility in the stent structure both in the compressed condition aswell as the expanded condition. Generally, the greater the longitudinalflexibility of the stent, the easier and more safely it can be deliveredto the target site, especially where the implantation site is on acurved section of the body lumen, such as a coronary artery or aperipheral blood vessel. The number of spines 76 formed by the collineararrangement of interconnecting elements 13 can vary from one to as manyas can be reasonably placed on a stent, however, for a minimal energystorage with a maximum flexibility, two to four spines are recommended.

[0064] As shown in FIG. 2, stent 10 serves to hold open artery 15 afterthe catheter body 14 is withdrawn from the artery and help reduce thelikelihood of restenosis. Due to formation of stent 10 from an elongatedtubular member, the undulating component of the cylindrical elements 12of stent 10 is relatively flat in transverse cross-section, so that whenthe stent is expanded, the cylindrical elements 12 are pressed into thewall of the artery 15 and do not result in an interference with theblood flow through the artery 15. Cylindrical elements 12 which arepressed into the wall of artery 15 will eventually be covered withendothelial cell growth which further minimizes blood flow turbulence.The serpentine pattern of cylindrical sections 12 provide good packingcharacteristics to prevent stent movement within the artery. Moreover,the closely spaced cylindrical elements 12 at regular intervals provideuniform support for the wall of artery 15. While FIGS. 1 and 2 depict avessel having an area of compressed plaque, the stent 10 can be used forpurposes such as repairing a detached lining in the artery, or to assistin attaching a vascular grasp (not shown) when repairing an aorticabdominal aneurysm.

[0065] The stent of the present invention can be made in many ways.However, the preferred method of making the stent is to cut athin-walled tubular member, to remove portions of the tubing in thedesired pattern for the stent, leaving relatively untouched the portionsof the metallic tubing which are to form the stent. It is preferred tocut the tubing in the desired pattern by means of a machine-controlledlaser.

[0066] Generally, the tubing is put in a rotatable collet fixture of amachine-controlled apparatus for positioning the tubing relative to alaser. According to machine-encoded instructions, the tubing is thenrotated and moved longitudinally relative to the laser which is alsomachine-controlled. The laser selectively removes the material from thetubing by ablation and a pattern is cut into the tube. The tube istherefore cut into the discrete pattern of the finished stent. Furtherdetails on how the tubing can be cut by a laser are found in U.S. Pat.Nos. 5,759,192 (Saunders) and 5,780,807 (Saunders), which have beenassigned to Advanced Cardiovascular Systems, Inc. and are incorporatedherein by reference in their entirely.

[0067] The process of cutting a pattern for the stent into the tubinggenerally is automated except for loading and unloading the length oftubing. For example, a pattern can be cut in tubing using a CNC-opposingcollet fixture for axial rotation of the length of tubing, inconjunction with CNC X/Y table to move the length of tubing axiallyrelative to a machine-controlled laser as described. The entire spacebetween collets can be patterned using the CO₂, Nd or YAG laser set-upof the foregoing example. The program for control of the apparatus isdependent on the particular configuration used and the pattern to beablated in the coding.

[0068] A suitable composition of Nitinol used in the manufacture of thestent of the present invention is approximately 55% nickel and 45%titanium (by weight) with trace amounts of other elements making upabout 0.5% of the composition. The austenite transformation temperatureis between about −15° C. and 0° C. in order to achieve superlastecity.The austenite temperature is measured by the bend and free recoverytangent method. The upper plateau strength is about a minimum of 60,000psi with an ultimate tensile strength of a minimum of about 155,000 psi.The permanent set (after applying 8% strain and unloading), isapproximately 0.5%. The breaking elongation is a minimum of 10%. Itshould be appreciated that other compositions of Nitinol can beutilized, as can other self-expanding alloys, to obtain the samefeatures of a self-expanding stent made in accordance with the presentinvention.

[0069] The stent of the present invention can be laser cut from a tubeof super-elastic (sometimes called pseudo-elastic) nickel titanium(Nitinol) whose transformation temperature is below body temperature.All of the stent diameters are cut with the same stent pattern, and thestent is expanded and heat treated to be stable at the desired finaldiameter. The heat treatment also controls the transformationtemperature of the Nitinol such that the stent is super elastic at bodytemperature. The transformation temperature is at or below bodytemperature so that the stent is superelastic at body temperature. Thestent is electro polished to obtain a smooth finish with a thin layer oftitanium oxide placed on the surface. The stent is usually implantedinto the target vessel which is smaller than the stent diameter so thatthe stent applies a force to the vessel wall to keep it open.

[0070] The stent tubing may be made of suitable biocompatible materialbesides super-elastic nickel-titanium (NiTi) alloys. In this case thestent would be formed full size but deformed (e.g. compressed) to asmaller diameter onto the balloon of the delivery catheter to facilitateintraluminal delivery to a desired intraluminal site. The stress inducedby the deformation transforms the stent from an austenite phase to amartensite phase, and upon release of the force when the stent reachesthe desired intraluminal location, allows the stent to expand due to thetransformation back to the more stable austenite phase. Further detailsof how NiTi super-elastic alloys operate can be found in U.S. Pat. Nos.4,665,906 (Jervis) and 5,067,957 (Jervis), incorporated herein byreference in their entirety.

[0071] The stent diameters are very small, so the tubing from which itis made must necessarily also have a small diameter. For PTCAapplications, typically the stent has an outer diameter on the order ofabout 1.65 mm (0.065 inches) in the unexpended condition, the same outerdiameter of the hypotubing from which it is made, and can be expanded toan outer diameter of 5.08 mm (0.2 inches) or more. The wall thickness ofthe tubing is about 0.076 mm (0.003 inches). For stents implanted inother body lumens, such as PTA applications, the dimensions of thetubing are correspondingly larger. This stent is also designed forcarotid applications, so the outer diameter of the tubing wouldtypically be about 0.095 inches with a wall thickness of about 0.007inches. The diameters of a carotid stent would be about 5-8 mm. While itis preferred that the stents be made from laser cut tubing, thoseskilled in the art will realize that the stent can be laser cut from aflat sheet and then rolled up in a cylindrical configuration with thelongitudinal edges welded to form a cylindrical member.

[0072] While the invention has been illustrated and described herein interms of its use as intravascular stents, it will be apparent to thoseskilled in the art that the stents can be used in other instances in allconduits in the body, such as, but not limited to, the urethra andesophagus. Since the stent of the present invention has the novelfeature of self-expanding to a large diameter while retaining itsstructural integrity, it is particularly well suited for implantation inalmost any vessel where such devices are used. This feature, coupledwith limited longitudinal contraction of the stent when it is radiallyexpanded, provide a highly desirable support member for all vessels inthe body. Other modifications and improvements may be made withoutdeparting from the scope of the invention.

What is claimed is:
 1. A self expanding stent having longitudinalflexibility for implanting in a body lumen and expandable from acompressed condition to an expanded condition, comprising: a pluralityof adjacent cylindrical elements made from a self-expanding material,each cylindrical element having a circumference extending around alongitudinal stent axis and being substantially independently expandablein the radial direction, wherein the plurality of adjacent cylindricalelements are arranged in alignment along the longitudinal stent axis andform a generally tubular member; and a plurality of interconnectingmembers extending between the adjacent cylindrical elements andconnecting the adjacent cylindrical elements to one another, whereinsome of the interconnecting members are aligned collinearly with respectto each other to form a continuous spine which extends along the lengthof the stent.
 2. The stent of claim 1 , wherein the cylindrical elementsare formed in a generally serpentine wave pattern transverse to thelongitudinal axis and contain alternating valley portions, peakportions, and double curved portions.
 3. The stent of claim 2 , whereinthe interconnecting members are connected at the double curved portionsof each cylindrically element.
 4. The stent of claim 1 , wherein theplurality of interconnecting members form a plurality of continuousspines which extend along the length of the stent.
 5. The stent of claim3 , wherein the plurality of interconnecting members form a plurality ofspines which extend along the length of the stent.
 6. The stent of claim1 , wherein said stent is formed of a biocompatible material such assuper elastic nickel titanium alloy.
 7. The stent of claim 1 , whereinthe stent is formed from a single piece of tubing.
 8. A stent deliverysystem comprising: a delivery catheter having an inner tubular memberhaving a region for mounting a compressed stent thereon and an outertubular member having a restraining sheath overlying said inner tubularmember and adapted for axial movement with respect to said inner tubularmember; a housing assembly having a pull-back handle slidably mounted ona base, said inner tubular member having a proximal end attached to saidbase and said outer tubular member having a proximal end attached tosaid pull-back handle whereby movement of said pull-back handleproximally retracts said restraining sheath proximally from thecompressed stent on the inner tubular member, which the inner tubularmember remains stationary.
 9. The stent delivery system of claim 8 ,wherein said inner tubular member includes a guide wire lumen extendingfrom the proximal end of the inner tubular member to the distal end ofthe inner tubular member.
 10. The stent delivery system of claim 8further including a lock mechanism for preventing the pull-back handlefrom moving proximally until the compressed stent is ready to bedeployed.
 11. The stent delivery system of claim 8 further includingmeans for evacuating air from the delivery catheter.
 12. The stentdelivery system of claim 11 wherein an annular space is formed betweenthe outer tubular member and the inner tubular member and furthercomprising an opening in the inner tubular member which is in fluidcommunication with the annular space and the guide wire lumen, whereinfluid may be introduced into the guide wire lumen through the opening inthe inner tubular member so that the fluid is introduced into annularspace and eventually flows through the distal end of the outer tubularmember.
 13. The stent delivery system of claim 8 wherein the base of thehousing assembly has a contoured shape to fit the contour of the leg ofa patient.
 14. The stent delivery system of claim 8 wherein the housingassembly has a thumb groove located where the proximal end of the basewherein a downward force is applied by the thumb of the user on thethumb groove to help keep the housing assembly stationary during stentdeployment.
 15. The stent delivery system of claim 8 wherein said innertubular member has a proximal portion and a distal portion, saidproximal portion being made from a compression resistant tubing.